How do I render and save as MP4?
Blender will render the frames that are between the start frame and end frame if you press Animation.

The outputfile is located in the path that is shown in the text-box. You can change it pressing the button with the folder icon

You can either the animation as an image-file sequence or as a video file. MPEG will output .dvd format, but you can change the extension to .mp4 in the windows explorer (you must check the option to view the file extension in folder options from windows